A planet-like object about eight times the size of Jupiter is locked in orbit around very young star, though the exact relationship between the two remains a mystery.
Circling 300 times farther from the star than Earth orbits the sun, the object could be a planet, perhaps one that was catapulted out into the nether regions after a collision or close encounter with an as-yet undetected sibling planet.
"There is no theory for how a true planet can form at 300 AU (astronomical units, with one unit equal to 93 million miles, the mean distance between Earth and the sun). It's not really a lack of imagination. It's a lack of physics," California Institute of Technology astronomer John Johnson told Discovery News.
Current theories, observations and computer models show that planets form from a disk of gas and dust that circles young stars. Less material is available as the distance to the parent star grows.
"You actually have to have material out there to have the planet forming," Johnson said.
Another option is that the object, designated as 1RXS J160929.1-210524 b and located about 500 light-years away in the constellation Scorpius, is a new type of failed star — something akin to a brown dwarf, though about half the size — which formed along with the primary star about 5 million years ago.
But scientists similarly are at a loss to explain how such a relatively small object could have survived the stellar birthing process.
"The models show it's difficult to successfully produce a low-mass object next to a much more massive object," said University of Montreal astronomer David Lafreniere, who lead a team that photographed the object in 2008, the first time a planet beyond the solar system was directly imaged.
Confirmation this week by Lafreniere and colleagues that the object is indeed in orbit with its parent — or sibling — star should galvanize follow-on studies that could illuminate the relationship, such as finding sister planets or building an inventory of similar systems elsewhere in the galaxy.
"This is a puzzle right now," Lafreniere said. "How do you end up with an object with this mass with this large of a separation?"
The research will appear in an upcoming issue of The Astrophysical Journal.